Pre-objective type light scanning units prevail as light scanning units for use in laser printers etc. In a pre-objective type light scanning unit, a scanning lens such as an Fθ lens is disposed in a stage subsequent to an optical deflection module such as a rotary polygon mirror. There is a problem that the size of the lens is large enough to complicate the configuration of the light scanning unit and increase the cost thereof.
In contrast, post-objective type light scanning units are expected as a solution for the aforementioned problem because a scanning lens is not disposed in a stage subsequent to an optical deflection module. However, in the post-objective type light scanning units, misalignment in imaging may occur due to a variation in a distance between the optical deflection module and a to-be-scanned surface in accordance with an angle of deflection of a light beam by the optical deflection module, that is, a scanning angle. It is therefore necessary to provide a means for correcting the misalignment.
According to JP-A-5-313089, a rotary polygon mirror is used as an optical deflector, and a reflective surface thereof is made aspherical enough to correct the curvature of field in a direction to which a light beam is deflected for scanning (hereinafter referred to as “main-scanning direction”). In addition, the refractive power of a correcting lens which is disposed between the optical deflector and a to-be-scanned surface is varied from its center to its periphery in a direction perpendicular to the main-scanning direction (hereinafter referred to as “sub-scanning direction”) so as to correct the curvature of field in the sub-scanning direction. It is, however, difficult to control the quality with which the aspherical reflective surface is manufactured. It is also troublesome that the correcting lens may have a large size in the main-scanning direction.
According to JP-A-7-72402, a range from a central portion to a peripheral portion can be placed within an allowable depth by use of a Bessel beam which is characterized by having a very deep focal depth. This method requires a Bessel beam generating module such as a conical lens. Thus the cost increases. In addition, there occurs a phenomenon that a secondary beam with a comparatively high intensity appears around a main beam. This phenomenon may cause a problem in a system having a high developability.
According to JP-A-7-244251, a reflective surface of a polygon mirror is formed into a flat shape, and a concave aspherical transparent member is provided in tight contact with the reflective surface so as to correct a curvature of field. There are conceivable problems as follows. That is, the manufacturing cost of the polygon mirror increases. In addition, it is difficult to control the profile irregularity of the aspherical transparent member and to control the uniformity of its internal refractive index.
According to JP-A-8-35937, a set of lenses are moved in an optical axis so as to correct a curvature of field. It is, however, believed that it is difficult in view of speed of response to move the lenses synchronously in scanning units.
According to JP-A-59-116715, a plurality of light sources are installed so that their optical axes are matched with one another by half mirrors. One of the light sources is selected in accordance with an angle of deflection of a light beam, that is, a scanning angle. Here, a plurality of half mirrors are used when 3 or more light sources are used. Accordingly, when a plurality of independent light sources are used, the light sources may change their positions relative to a common lens independently of one another. Thus, relative misalignment may occur among scanning lines on a to-be-scanned surface.
Each half mirror transmits one of light beams and reflects the other. When the surface of the half mirror is inclined, a deflection angle twice as large as the inclination of the surface may be produced in the reflected beam though there is no change in the deflection angle of the transmitted beam. Particularly when a light source forming a scanning line is switched to another to form another scanning line, there arises a problem that discontinuity between the scanning lines can be recognized in the portion where the light source is switched. Further, in a system using a plurality of half mirrors, which is described as means using three or more light sources, the aforementioned misalignment is doubled. The system is not practical.
Patent Document 1: JP-A-5-313089
Patent Document 2: JP-A-7-072402
Patent Document 3: JP-A-7-244251
Patent Document 4: JP-A-8-035937
Patent Document 5: JP-A-59-116715